The European aspen (Populus tremula) is thought to reproduce mostly asexually. Thus aspen forms clones, in which several ramets belong to one genetically defined genet. We compared the clonal structure of aspen in old-growth and managed forests in southern and northeastern Finland. Clones were identified using morphological characters and nine microsatellite loci originally developed for Populus tremuloides. There were more clones identified by microsatellites than morphotypes both in old-growth and managed forest. The average size of the clones was only 2.3 ramets and most clones (70%) consisted of just one ramet. The size of the clones showed no difference between managed and old-growth forests or between northeastern and southern Finland. The small size of the clones suggests that most of them are relatively young. Therefore, sexual reproduction may be more common than previously thought. There was an aggregated spatial genetic structure as measured by Moran's I (0–10 m) and by co-ancestry (ρij, 0–20 m). Low level of co-ancestry can be explained by relatively unrestricted gene flow, the important role of disturbance in reproduction, and/or local selection.
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